It is well known to manufacture ceramic glow plugs having a multi-layered construction. Examples of such conventional glow plugs are described in U.S. Pat. Nos. 4,742,209, 5,304,778 and 5,519,187. In general, these glow plugs have a ceramic heater with a conductive core enclosed by insulative and resistive ceramic layers, respectively. The layers are separately cast and fitted together. The resulting green body is then sintered to form a ceramic heater. Such ceramic heaters suffer several drawbacks. Used in a glow plug, they experience cyclic heating and cooling, which results in high internal stresses at the interfacial junction between the ceramic layers, promoting eventual failure of the glow plugs. To reduce this failure rate, such ceramic heaters tend to be cycled at lower temperatures than would be optimal in a diesel engine.
The internal stresses of a layered glow plug are mainly the result of differences in the coefficients of thermal expansion between the differently composed layers. The different layers of the glow plug expand and contract at different rates. Further, residual stresses are the result of manufacture, particularly from uneven contraction in the cooling period which occurs below the plastic deformation state of the ceramic composition, and from non-uniform attachment between the layers.
A ceramic heater that has reduced internal stress is described in U.S. patent application Ser. No. 08/882,306, filed Jun. 25, 1997. This application discloses a ceramic heater that is slip cast as a unitary body with a graduated composition in the interfacial boundary zones. While the ceramic heater described in this application has reduced internal stresses, it has been found to be difficult to manufacture to the stringent standards required of such heaters. In particular, the layer thicknesses are difficult to control precisely, and even minor discrepancies can lead to widely varying heat output in the final heater. Precise control of heating characteristics, and limiting heating losses in the base portion of the heater element, is important if the ceramic heaters are to be mass produced for vehicle and engine manufacturers.
Additionally, it has been found that the performance of prior ceramic heater elements can be affected by moisture.
It is, therefore, desirable to provide a ceramic heater element that overcomes the disadvantages of the prior art. In particular, it is desirable to provide a ceramic heater element that has low internal thermal stresses, precisely controllable and reproducible heating characteristics that are focussed mainly to the heating tip of the element and that is more resistant to the effects of moisture.